Problem: Which of these numbers is prime? ${12,\ 37,\ 57,\ 81,\ 85}$
A prime number has exactly two factors: 1 and itself. The factors of 12 are 1, 2, 3, 4, 6, and 12. The factors of 37 are 1 and 37. The factors of 57 are 1, 3, 19, and 57. The factors of 81 are 1, 3, 9, 27, and 81. The factors of 85 are 1, 5, 17, and 85. Thus, 37 is a prime number.